A fuel cell converts chemical energy directly into electric energy by supplying a fuel and an oxidant to two electrically-connected electrodes and thus causing electrochemical oxidation of the fuel. Unlike thermal power generation, a fuel cell is not limited by the Carnot cycle; therefore, it shows high energy conversion efficiency. A fuel cell is generally constituted of a stack of single cells, each of which comprises a membrane electrode assembly as the basic structure, in which an electrolyte membrane is sandwiched between a pair of electrodes.
Noble metal catalysts such as a platinum catalyst and a platinum alloy catalyst have been used as the catalyst of the anode and cathode electrodes of a fuel cell. However, noble metal catalysts are scarce resources and it is expensive to use them for large-scale commercial production of fuel cells.
Meanwhile in noble metal catalyst particles, catalytic reaction occurs on the surface of the particles only and the inside of the particles seldom participates in catalytic reaction. Therefore, the catalytic activity per unit mass of a noble metal catalyst particle is not always high.
Catalyst particles having such a structure that a core particle is covered with an outermost layer, that is, a so-called core-shell structure, are known as a technique which can increase the catalytic activity per unit mass of a noble metal catalyst. Catalyst particles having a core-shell structure can secure catalytic activity and cost reduction by using a material with excellent catalytic activity (such as noble metal) as the outermost layer and a relatively inexpensive material which does not directly participate in catalytic reaction as the core particle.
As the method for producing catalyst particles having a core-shell structure, there may be mentioned a method disclosed in Patent Literature 1. The method disclosed in Patent Literature 1 is a method for producing an electrode catalyst for fuel cells, comprising the steps of using specific particles as the core portion and covering the core portion with a shell portion. In Patent Literature 1, it is explained that filtration, washing, etc., of core-shell type metal nanoparticles can be conducted after the step of covering the core portion with the shell portion, and suction filtration with water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like is mentioned as a concrete method of the filtration and washing.
Patent Literatures 2 and 3 disclose methods for producing a catalyst for fuel cells, which are not the method of producing catalyst particles having a core-shell structure.